1. Field of the Invention
This invention relates generally to a stepped structure and especially to folding stepladders, stepstools and stairways.
In particular, the device of this invention concerns a portable free-standing spiral staircase having swingable steps.
2. Brief Description of the Prior Art
The developmental evolution of the structural design for household or domestic stepstools and folding ladders has not produced significant advancement over the basic concept found in the prior art.
The most commonly known of these devices included the self-supporting folding stepladder which was designed with two opposite side upright members having transverse steps extending therebetween. A top platform was usually provided at the uppermost ends of the upright members. The rear props were pivotally connected to the upright members and the load distribution was thus concentrated at the four points at which the pair of upright members and the rear props rested upon a surface. A typical ladder of this type is illustrated in U.S. Pat. No. 3,139,155.
There were several problems inherent with that hinged stepladder. From the safety point of view, if the rear props were not fully extended or if each of the four legs did not rest on a level surface, the ladder would be unstable. A further disadvantage of those prior art foldable stepladders was that they were rather heavy and cumbersome to use and in many instances of domestic usage such as in a kitchen, they presented a storage problem.
The portable spiral staircase of this invention, in contrast, provides a continuous bearing surface in place of the previous four point contact for greater stability. In addition, this device provides a safety positive locking engagement for the helical steps when in the operative mode.
Previous attempts to provide spiral step arrangements such as shown in U.S. Pat. No. 310,226 have produced cumbersome structures having hinged steps which were not nestable but rather laid in parallel planes. In addition, multiple pivots were employed and a hand support was not present.
It should be readily apparent that the spiral staircase of this invention provides a distinct improvement over those prior art devices and further includes additional advancements for added safety, convenience, multifunctional uses and compact storage.